1. Field of the Invention
The present invention relates to a single-phase brushless motor, and more particularly, to a control technology of a voltage waveform which is applied to a coil.
2. Description of the Related Art
Brushless motors include a multi-phase brushless motor in which a coil is configured to be driven using a plurality of phases and a single-phase brushless motor in which a coil is configured to be driven using a single phase. Particularly, the single-phase brushless motor having a simple structure is used in a fan driving motor or the like which does not demand a high rotation accuracy. In the single-phase brushless motor, an energizing direction is switched for each 180° of an electrical angle such that a torque is generated. In this case, for example, as shown in FIG. 1A, generally, control of the applied voltage is also performed by a rectangular wave energization in which a voltage is applied through repetition of a simple rectangular wave.
When the rectangular wave energization is performed, as shown FIG. 1B, it has been observed that distortion is generated in a waveform of current which flows in the coil. Particularly, before and after the energizing direction is switched, among rotor magnetic fluxes, the absolute value of a magnetic flux component which is interlinked with the coil becomes maximal, and a counter electromotive force which is generated in the coil becomes minimal. Thereby, the amount of current tends to be relatively increased.
The current which flows in the coil is divided into a component which generates torque and a component which changes the magnetic flux. In this case, from a positional relationship of the rotor with respect to a stator, the component which changes the size of the magnetic flux becomes dominant as the current, which flows in the coil in the vicinity before and after the energizing direction, is switched. Moreover, in the vicinity (electrical angles 90° and 270°) of an electrical angle is rotated by 90° from a position, the component which generates the torque becomes dominant as the current flows in the coil. The current, which flows in the coil in the vicinity before and after the energizing direction is switched, does not contribute very much to the generation of the torque. Thereby, before and after the energizing direction is switched, an increase in the amount of current which flows in the coil generates a decrease in the motor efficiency. Moreover, a sudden change in the amount of current becomes a cause of noise or vibration.
Therefore, for example, in JP-A2004-88870 and JP-A2007-174778, a so-called soft switching is disclosed, which is a technology which gradually changes the applied voltage before and after the switching of the energizing direction by using PWM (Pulse Width Modulation) control to suppress the increase in the amount of current before and after the switching of the energizing direction.
By using the soft switching, the increase in current before and after the switching of the energizing direction can be suppressed. However, since the suppression effect is partial, there is room for improvement.